Not European but I'll take a guess. Hope this helps:
In the UK (and maybe elsewhere in Europe) a "garden" is what people in the US would call a "yard"; an open space near a residence. Often, but not necessarily covered in grass or plants.
"Garden paving" would mean covering it with something impervious to rain, such as concrete or asphalt.
So a "garden pavement tax" would be a penalty for people who choose to cover up soil with an rain-blocking material, to encourage them to use an alternative which allows the ground to soak up the rain.
As an example, in my town there is a public park in a flood plain, its driveways are made with "permeable pavers"[1] which are basically thick brick tiles with holes in them. They allow the rain to go through to the ground instead of building up and flooding the parking lot.
[1]: https://duckduckgo.com/?q=permeable+paver&iax=1&ia=images